Rate of occurrence measurements based upon the observation of discrete events are useful in many fields. In medicine and health care, for example, knowledge of a patient's heartbeat rate, respiration rate, and intravenous solution drip rate are all important parameters commonly monitored as indica of the condition and treatment efficiency. In time-motion studies, the rate at which various manufacturing operations are performed is central to many analyses.
Conventional methods of making rate measurements are less than satisfactory particularly when used by process or medical personnel who cannot be burdened with complex instrumentation. In the usual approach, an observer with a time piece counts the number of occurrences in a given time interval, e.g., the number of heartbeats in sixty seconds. The disadvantage of this approach, however, is that it is inaccurate and time-consuming. Its accuracy is limited to about .+-.0.4 events in any given time interval, and little perceptible information is obtained until expiration of the chosen time interval. In addition, the observer's attention is diverted to the measurement function for a long time period and often a mental calculation is required at the end of the observation.
Alternative approaches involve the use of dedicated instruments specifically designed to detect and count the occurrence of a specific event. Aside from their inherent inflexibility, these instruments also are relatively slow and provide no information until the expiration of a chosen time interval, or until after the occurrence of a specified number of events.